Fiber optical systems are used in optical telecommunications to send optical signals from one location to another. Fiber optical systems typically employ an optical fiber cable that defines an optical link that optically connects one piece of optical equipment at one location (i.e., on one side of the optical link) to another piece of optical equipment at another location (i.e., on the other side of the optical link). For example, the optical link may be between a server in a data center and a computer at a control center. In many cases, the optical fiber cable supports multiple optical fibers over which the optical signals are transmitted and received in both directions using transmitters and receivers (e.g., transceivers) at both ends of the optical link.
Fiber-based loopback apparatus are often used to test the performance of fiber optical systems by providing an optical communication path over at least a portion of the optical link between a transmitter and a receiver at the same piece of optical equipment, i.e., on the same side of the optical link. This allows for the performance of the fiber optical system to be evaluated and managed from one end of the system. The loopback apparatus simulates the actual losses in the optical transmission than can occur in a fiber optical system by providing a select amount of attenuation to the optical signals. The attenuation is provided by specially doped fibers caned “attenuation fibers” that provide relatively large amounts of attenuation over a relatively short length. A desired amount of attenuation is accomplished by selecting an attenuation fiber with a select length.
Fiber-based loopback devices have several drawbacks. First, the attenuation range is relatively narrow due to restricted choices of the attenuation fiber. In particular, there are limits to the amount of attenuation that can be achieved using fiber doping and reasonable lengths of attenuation fiber, especially for multimode fibers. Second, attention fiber is expensive. Third, the ability of fiber-based loopback apparatus to only provide different amounts of attenuation limits the ability to perform other desirable optical signal performance evaluations and management.